The field of the invention relates generally to electronically controlled motors, and more specifically, to methods and systems for providing combined blower motor and draft inducer motor control.
In a conventional furnace, natural convection currents move air through the exhaust outlet of the furnace's combustion chamber to carry away exhaust gases. The moving air further induces a draft in the combustion chamber for mixing oxygen with the fuel being burned in the chamber. Heat energy remaining in the exhaust gases, however, is lost to the atmosphere which decreases the overall efficiency of the furnace. The use of heat exchangers improves furnace efficiency by extracting additional heat from the exhaust gases before they are vented to the atmosphere. Extracting heat from the exhaust gases, however, reduces the natural convection currents which would otherwise carry the gases away. One solution has been to use a draft inducing fan to force the exhaust gases into the atmosphere. The draft inducing fan is run by a motor. Such a motor is referred to herein as a draft inducer motor.
In certain conventional furnace applications, fan (draft inducer motor) speed is controlled, and it is known to control motor speed as a function of sensed pressure. In other furnace applications, it is known to control this motor speed by sensing ignition in the combustion chamber, since combustion decreases the density of the combustion chamber gases moved by the draft inducer fan which can also affect speed/torque of the draft inducer motor and fan combination. Finally, different furnaces that incorporate draft inducer motors have different restrictions to air flow and require the draft inducer motor to operate at different speeds and/or torques to produce the desired pressure in the combustion chamber.
Most conventional furnaces comprise what are referred to as forced air systems and operate by forcing heated (or cooled) air through a duct system. The motor that forces the air through the duct system is generally referred to as a blower motor. Air flow through this “indoor section” of a heating, ventilating and/or air conditioning (HVAC) system generally determines several key performance characteristics of such a system, including, but not limited to, total system capacity for cooling and dehumidification, latent system capacity for dehumidification, overall efficiency (capacity/input power), and indoor sound level. Variations in the rate of air flow caused by a blower motor can cause these performance characteristics to vary differently. As such, systems exist to control the speed of blower motors.
However, most conventional systems provide only a single air flow rate for cooling and another for heating. Even in systems having a multispeed induction motors, the installer often selects one of the speed taps to operate the motor at single speeds for cooling or heating. More advanced HVAC systems may provide variable speed motors but require a separate controller, such as a humidistat, to switch motor speed.
Electronically controlled variable speed motors (VSMs), including electronically commutated motors (ECMs) and induction motors are generally used in air handling applications in HVAC systems, for both the draft inducer application and the blower motor application. As such, controllers are utilized for each of these motors.